In the event in Mesetas, a small central town in an area that was once the group's stronghold, Timochenko said the Farc would continue to exist, but in a different legal and democratic form as it enters a new phase in its history.

They are expected to change their name, as the long-standing acronym stands for Revolutionary Armed Forces of Colombia.

"Today, we are not letting Colombia down. Today, we are laying down our weapons," he said.

President Santos, who won a Nobel Prize for his efforts in securing the agreement, hailed it as a "very special day" and "the day weapons became words."

'Cultivating peace': By Natalio Cosoy, BBC News, Mesetas, Colombia

Image copyrightAFP

The fact that more than 9,000 Farc fighters and militiamen are not armed any more represents a crucial landmark in the implementation of the peace process between the government and the group.

President Santos said this was the best news for Colombia in 50 years. He gave an AK-47 rifle that an artist had transformed into a shovel as a present to Timochenko to "cultivate peace".

When Timochenko spoke, he said that the Farc had not failed Colombia, because they had handed over their weapons.

After his speech, hundreds of small white butterflies were released. Hundreds of Farc members and dozens of government officials were watching the ceremony here. Most of the rebels and officials were wearing white shirts and t-shirts, a symbol of peace.

United Nations monitors in the country said on Monday that the arms handover had been completed a day ahead of a revised schedule. The weapons have been registered and boxed away, they added.

Speaking at the ceremony, Jean Arnault, the head of the UN's mission in Colombia, said the disarmament process was an example to the rest of the world.

The monitors said they had also found and emptied 77 out of the Farc's 900 arms caches hidden around the countryside.

Image copyrightAFP / Getty Images Image caption
"All for peace" is written on the side of one of the FARC rebels' tents in Mesetas

However, US monitors have warned that the task of locating and emptying the Farc's remaining caches could take many months because of the difficulty in reaching them.

Some of them are reachable only by river or foot through dense jungle and mountain terrain. Many are booby-trapped and need to be painstakingly dismantled.

Image copyrightAFPImage caption
UN monitors have been listing and registering all weapons handed in by the Farc

Many Colombians are concerned that if there is any delay, these weapons could fall into the hand of criminals, paramilitaries or the ELN.

There are also worries over the fate of territory and drug-trafficking operations that were previously controlled by the Farc.